1. Field of the Invention
The invention is related to the field of electrical resistivity well logging apparatus and methods. More specifically, the invention is related to apparatus and methods for determining the resistivity of earth formations in each one of a plurality of azimuthal segments around the circumference of a wellbore.
2. Description of the Related Art
A number of different types of well logging instruments are well known in the art for measuring the electrical resistivity of earth formations from within a wellbore drilled through these earth formations. These instruments include induction and galvanic (electrode) type devices. Most of the electrical resistivity well logging instruments known in the art, however, only measure variations in the resistivity axially with respect to the wellbore. The resistivity well logging instruments known in the art generally make measurements which reflect the assumption that the resistivity is azimuthally invariant with respect to the axis of the wellbore. Therefore these instruments typically do not provide any indication of azimuthal variations in the resistivity of the formation around the axis of the wellbore.
One instrument which can provide indications of azimuthal variation in the formation resistivity is described in U. S. Pat. No. 5,426,368 issued to Benimeli et al. This instrument includes a plurality of azimuthally segmented electrodes through which an electrical measuring current is passed. The measuring current is constrained generally to two distinct paths. The first path includes a return position at an electrode on the body of the instrument, this electrode being axially proximal to the azimuthally segmented electrodes so that most of the current in the first path flows through the fluid filling the wellbore. The second path is generally radially outward from the segmented electrodes so that the second current path includes both the wellbore and includes the earth formations through which the wellbore is drilled. The amount of the measuring current which passes through each of these electrodes, and the voltage drop associated with the measuring current flowing along the first path are used to calculate a fractional amount of the total resistivity derived signal which originates in the wellbore. The voltage drop associated with the second path includes effects of current flow in the wellbore and effects of current flow in the earth formations. By providing paths both including and excluding the effects of current flow in the earth formation, this instrument provides a means to account for the effects of irregular shape of the wellbore and/or eccentering of the instrument within the wellbore upon the individual azimuthally segmented measurements of resistivity.
The instrument disclosed in the Benimeli et al '368 patent, however, is subject to error in the azimuthally segmented resistivity measurements when the wellbore shape is highly irregular. This instrument is also subject to measurement error when the conductivity of fluid in the wellbore is low relative to the conductivity of the earth formations, because the voltage drop measurements used to determine the shape of the wellbore tend to be large relative to the voltage drop measurements from the earth formation under these conditions. Further, the instrument disclosed in the Benimeli et al '368 patent requires complex circuitry in order to measure two complete sets of current and voltage drop signals for each of the two current paths associated with each azimuthally segmented electrode.
The invention is intended to provide a way to measure azimuthal variations in the resistivity of earth formations with minimal effect of irregular wellbore shape and instrument eccentering. The invention is also intended to provide azimuthally segmented measurements of formation resistivity using greatly simplified electronic circuitry.